As described in co-pending patent application Ser. No. 10/935,986 filed Sep. 8, 2004 by Steven Greelish entitled “Method and Apparatus For Performing an Ultrasonic Survey,” incorporated herein by reference, a system is provided for surveying a ship's hull by accurately ascertaining the position of the ship's hull relative to, for instance, a robotic sensor flying around the hull. In this system a coordinate system is established so that the position of the robot relative to the hull can be ascertained, thus to permit acoustic hull measurements.
In hull inspections, the purpose is to determine erosion or wasting of the hull so as to be able to certify the seaworthiness of the ship. Determining seaworthiness depends upon accurate measurements of the thickness of the hull, and even hull plating and coatings or paint, with coating thickness usually measured in thousandths of an inch. In order for the ship to be certified as seaworthy, classification societies use hull thickness information to determine the seaworthiness of a ship. However, these societies require a certain level of calibration that ensures that the measurements obtained are accurate.
In the past, in order to convince classification societies of the accuracy of the measurements, it has been the practice to obtain a sample of the ship's hull and to calibrate the instruments either away from the ship, or to send a diver down with a test plate for the calibration process.
While high accuracy thickness measurements can be made using high quality, high frequency instruments, it is a requirement for these instruments that the sensor be attached to the hull. This is impractical for in-water surveying systems and thus a lower frequency system is required so that the acoustic energy can penetrate the water column and then the hull. However, lower frequency systems result in degraded performance.
Moreover, using test samples that are transported to the test location at the hull results in a calibration system that suffers from changes in temperature, instrument damage and calibration shifts as one transports the test sample and/or instrument from one site to the other. Moreover, as mentioned above the thickness measurements can suffer degradation in terms of the frequency that is used, with the lower frequencies providing less accurate results.
There is another problem with the way in which acoustic thickness measuring instruments are calibrated and that is that the measurements themselves generally measure the timing of the receipt of a reflected pulse by integrating the return signal and measuring the position of the resulting signal envelope. These integrating systems suffer from the fact that they are amplitude-dependent, meaning that the timing of the return pulse as measured by the envelope shifts depending on the amplitude of the returned signal. Thus, any system that uses integration, or rectification and integration, suffers from the fact that the actual measurement is affected by the amplitude of the signal.
The result is that, for some of these hull-measuring systems, the thickness measurement may be off by 10 or 15%.
There is thus a requirement to be able to measure the hull thickness to within one-half of one percent, which translates in some cases to measurement accuracy better than one one-thousandth of an inch.
Also, a problem exists as to where exactly on the ship's hull the measurement is made. Locating the precise position on the hull is oftentimes difficult for divers, since they are oftentimes in brackish or clouded water adjacent to the hull. Therefore, there is a requirement to be able to identify exactly where on the hull the thickness measurement is being made.